Conventionally, a motor control apparatus has been known that detects a fault of an R/D converter (for example, Japanese Laid-open Patent Publication No. 9-72758). The motor control apparatus includes a resolver. Based on a reference signal having a prescribed cyclic waveform, the resolver outputs a sine signal and a cosine signal depending on the rotation angle of the motor. The R/D converter calculates the rotation angle of the motor based on the sine signal and the cosine signal output by the resolver. An A/D converter applies A/D conversions to the sine signal and the cosine signal output by the resolver at a peak timing of the reference signal.
The motor control apparatus includes a control unit that receives as input a signal representing the rotation angle of the motor output by the R/D converter, and the sine signal and the cosine signal output by the A/D converter, after having the A/D conversions applied to the sine and cosine signals. The control unit compares the motor rotation angle based on the output of the R/D converter, with a motor rotation angle calculated based on the sine signal and the cosine signal after the A/D conversions based on the output of the A/D converter, to detect a fault of the R/D converter. If the difference between the motor rotation angles is not over a threshold, the control unit determines that the RID converter is in a normal state; and if the difference between the motor rotation angles is over the threshold, the control unit determines that the R/D converter is in a faulty state.
Incidentally, in a motor control apparatus, a motor control process is a feedback control process in which a switching command is issued to a switching element of an inverter, based on a difference between a command current to the motor, and a current actually flowing in the motor. To execute such feedback control, it is necessary to apply an A/D conversion to the current flowing in the motor by an A/D converter. Thereupon, one may consider using the same A/D converter for applying an A/D conversion to the motor current, and for applying an A/D conversion to a resolver output signal that is used for determining whether the R/D converter is normal or faulty. Configured in this way, the motor control apparatus does not need to have an A/D converter for motor current conversion, and a separate A/D converter far a resolver output conversion. Therefore, the circuit size can be reduced.
Also, to apply an A/D conversion to the motor current used in the motor control process, it is necessary to obtain the motor current at a timing in a feedback cycle synchronized with the rotational speed of the motor. On the other hand, to apply A/D conversions to the sine signal and the cosine signal output by the resolver, it is necessary to obtain these signals at a peak timing of a reference signal. However, if the timing to apply the A/D conversion to the resolver output signals (namely, the peak timing of the reference signal) overlaps with the timing to apply the A/D conversion to the motor current (namely, the control timing following the feedback cycle of the motor control process), then, the motor control process based on data after the A/D conversion may not be executed appropriately, or an erroneous determination may be obtained by a fault detection process of the R/D converter.
In view of the above, it is a general object of at least one embodiment to provide a motor control apparatus that can apply A/D conversions to the resolver output signals and the motor current, respectively, by a common A/D converter while executing the motor control process appropriately, and avoiding an erroneous determination of a fault of the R/D converter.